Cluster bombing

THE outbreak of Ebola fever in Guinea, Liberia and Sierra Leone, which has killed more than 11,000 people, has dropped out of the news as it has been brought under control. Although new cases are now measured in dozens, rather than hundreds, a week, the disease has not been stamped out—and a new epidemic could flare up somewhere else at any time. A vaccine against the virus responsible would be of enormous value. And a paper in the Lancet suggests one is now available.

The vaccine, developed by the Public Health Agency of Canada and called rVSV-ZEBOV, smuggles one of the Ebola virus’s coat proteins into a person’s body in a Trojan horse called a vesicular stomatitis virus. This horse-and-cattle virus does not cause human illness, but its presence is enough to activate the immune system, which learns to recognise and react to the Ebola coat protein—and thus, the vaccine’s inventors hope, to clobber Ebola if it should encounter it.

The trial that the Lancet reports was conducted on more than 7,600 people in Guinea by a group of researchers led by Marie Paule Kieny of the World Health Organisation and John-Arne Rottingen of the Norwegian Institute of Public Health. It involved a procedure called ring vaccination, in which clusters of people who were particularly at risk were offered the chance to be vaccinated. A cluster included everyone who had been in contact with a confirmed victim of Ebola, and everyone who had in turn been in contact with one of those people. Most adults in each cluster were eligible for vaccination (the exceptions were women who were pregnant or breast-feeding) and the majority volunteered. Each cluster was assigned, at random, to one of two groups, known as arms. Those in one arm were offered the vaccine immediately. Those in the other were offered it after a three-week delay.

Ebola’s incubation period is ten days. In the case of those in the arm in which vaccination was delayed, there were 16 cases between the moment ten days had elapsed and the moment, 11 days later, when participants were vaccinated. No one who had been vaccinated in either arm of the trial contracted the disease once ten days had subsequently passed. The vaccine, in other words, seemed 100% efficient.

In light of this result, every new cluster involved in continuing trials will receive the vaccine immediately. If it continues to prove efficacious, approval for general use will no doubt follow, and the vaccine will spread to the other two affected countries. That may end the epidemic for good.